The present invention relates to a method and an apparatus for operating a cooling fluid circuit of an internal combustion engine for an automotive vehicle. The cooling fluid circuit includes a heat storage and a heat exchanger for a cabin heating system. The heat storage may be any heat storage which can be unloaded by a flowable medium; preferably the heat storage is a cooling fluid storage which can be loaded and unloaded by exchange of the cooling fluid.
Such heat storages which store waste heat of the engine have become known for quite some time. They may be used at cold start of the engine for heating the heating air for the vehicle cabin and/or for heating the engine to reduce exhaust gas emissions, see for example, "BWK Brennstoff Warme Kraft", Vol. 43 (1991) No. 6, pages 333-337.
In practice operation of such heat storages is generally such that when the heat storage is being unloaded the cooling fluid (cooling water) flows initially from; the heat storage through the heating system heat exchanger and thereafter through the engine and finally back into the heat storage in order to heat during cold start of the engine both the heating air and the engine as rapidly as possible. When the heat storage has been unloaded the system will be switched from the "storage heat supply mode" to the "engine heat supply mode". The terms "storage heat supply mode" and "engine heat supply mode" mean that the cooling fluid circuit communicating with the heating system heat exchanger is being supplied with heat from the heat storage and, respectively, directly from the engine.
The cooling fluid circuit usually includes--additional to the cooling fluid pump (water pump) driven by the engine--an electrical additional pump which provides for a high flow rate of the cooling fluid during cold start of the engine. A high flow rate of the cooling fluid flow is required in particular in order to enable operation of the heating system heat exchanger under so-called saturation conditions even at low engine speeds. Saturation conditions of the heat exchanger mean that a further increase of the flow rate of the cooling fluid flow from e.g. 600 l/h does not result in a significant increase of the temperature of the heating air. Such a high flow rate of the cooling fluid flow is required in particular by so-called cross-flow heat exchangers generally used in automotive vehicles. Cross-flow heat exchangers are of a design such that short air flow paths result in order to minimize pressure losses of the required air flow at the required surface areas of the heat exchanger at minimal space.
Operation of the heating system heat exchanger under saturation conditions has become usual in the field of automotive vehicles because this allows to reach the maximal heating air temperature by the cooling fluid discharged from the engine and gradually being heated from ambient temperature to operative temperature. This is why at cold start of the engine the high flow rate of the hot cooling fluid discharged from the heat storage results in maximal heating air temperatures within the heat exchanger operating under saturation conditions and a rapid substantial increase of the temperature of the heating air when the heat storage initially is being unloaded. On the other hand high flow rates of the cooling fluid flow cause the heat storage to be unloaded rapidly. As a result the temperature of the heating air initially increases very rapidly to a relatively high value such as e.g. 45.degree. and thereafter rapidly drops to the level of the temperature of the engine which at the time is still relatively low (e.g. 20.degree.), until the temperature of the heating air will again be raised gradually due to the engine heating up. The above mentioned temperature drop of the heating air is considered to be disturbing for the vehicle occupants; for comfort reasons it is desirable to maintain the temperature of the heating air after its rapid increase at a substantially uniform level until the engine will have reached a temperature sufficient for maintaining or increasing such temperature level.
In order to solve this problem one might consider to enlarge the heat storage or to delay unloading of the heat storage by mixing the cooling fluid discharged from the storage with cold cooling fluid discharged from the engine. Both solutions, however, are not satisfactory.
While mixing of hot cooling fluid from the heat storage and cold cooling fluid from the engine increases the time required to unload the heat storage, it results in proportionally reduced heating air temperatures.
Enlargement of the heat storage is not possible in most cases due to the limited space available in automotive vehicles. Furthermore, there may be problems of reloading the heat storage when the automotive vehicle is moved only short distances such as in city traffic.